In order to maintain powerful competitiveness of the 3rd Generation Partnership Project (3GPP for short) in the field of mobile communications, the 3GPP is currently devoting itself to researching on the evolution of a Packet Switched Core (PS Core for short) network and a Universal Mobile Telecommunication System Radio Access Network (UTRAN for short), with the purpose of enabling the evolved PS Core (EPC for short) to provide higher transmission rate and shorter transmission delay and support mobility management among Evolved UTRAN (E-UTRAN for short), GSM EDGE radio access network (GERAN for short), the UTRAN, the Wireless Local Area Network (WLAN for short) and other non-3GPP access networks, wherein, the GSM is Global System for Mobile communications and the EDGE is Enhanced Data Rate for GSM Evolution. This evolved mobile communication system is referred to as the Evolved Packet System (EPS for short).
FIG. 1 illustrates a diagram of architecture of enabling a CS service to fall back to a CS network when a terminal accesses an EPS network according to the prior art, and the various entities in FIG. 1 are illustrated as follows:
User Equipment (UE for short) 101;
UTRAN 102: a radio access network of a GSM/Universal Mobile Telecommunications System (UMTS for short) network, which provides circuit and packet radio access resources;
E-UTRAN 103: evolved radio access network, which can provide higher uplink and downlink rates, lower transmission delay and more reliable radio transmission, in the E-UTRAN, a network element Evolved NodeB (eNodeB for short) is included, which provides radio resources for accessing by the terminal;
Serving GPRS Supporting Node (SGSN for short) 104; Serving Gateway (S-GW for short) 105, which is one entity of the user plane, and is responsible for data routing and processing of the user plane;
Mobility Management Entity (MME for short) 106, which is one entity of the control plane, i.e., a server which temporarily stores user data, is responsible for managing and storing UE contexts (for example, UE/user identities, mobility management statuses, user security parameters etc.), allocates temporal identities to the user, and is responsible for authenticating the user when the user resides in the tracking region or the network;
Mobile Switching Center (MSC for short) 107, which can be a new functional entity MSC Server etc. in and after the R4 stage, and is represented by MSC/Visitor Location Register (VLR) uniformly in the present document;
Public Switched Telephone Network (PSTN for short) 108, which refers to a fixed telephone network here.
When the UE accesses the EPS network, the MME is notified to perform joint registration, and is notified of the capability of supporting CSFB, the MME substitutes the UE to initiate the joint registration with the MSC, the MSC receives joint registration message, and if there is no user data, the user data is acquired from a Home Location Register (HLR for short).
FIG. 2 illustrates a calling flowchart of enabling a CS service to fall back to the CS network when a terminal accesses an EPS network in the prior art. The UE has been registered with the MME and the MSC in the EPS network, at this time, the UE initiates a CS service, the UE transmits enhanced service request message carrying a CSFB indication to the MME, the MME notifies the UE to fall back to the UTRAN network to perform CS service through the E-UTRAN after receiving the enhanced service request message, which comprises the following steps:
in step 201, the UE transmits an enhanced service request carrying a CSFB indication to the MME, wherein, the CSFN indication is used to indicate the MME to perform circuit switch service fallback;
in step 202, the MME transmits CSFB notification message carrying a CSFB indication to the E-UTRAN to notify the E-UTRAN that the local UE needs to fall back to the UTRAN network;
in step 203, the E-UTRAN can ask for a measurement report from the UE, and decide a specific cell to which the UE falls back according to the measurement report;
in step 204, if there are data services for the UE in the EPS network, the E-UTRAN triggers a packet domain switching process, to switch the data services to the UTRAN network, which will not be described in detail by the present document;
in step 205, if there is no data service for the UE in the EPS network, the E-UTRAN notifies the UE to perform fallback to be switched to the UTRAN network;
in step 206, after the switching is successful, the UE transmits service request message to the MSC of a serving target access cell in the UTRAN network;
in step 207, if the above MSC is an MSC with which the UE jointly registers when accessing the EPS network, the MSC makes determination according to the service data of the user, receives a service request of this time, returns service access response message to the UE, and there exist encryption and authentication processes subsequently, which will not be described in detail by the present document;
in step 208, if in step 206, the MSC is not an MSC with which the UE jointly registers when accessing the EPS network, there is no user data for the MSC, and the MSC returns service access rejection message to the UE to notify the UE to initiate a location update process;
in step 209, the UE reinitiates a location update process in the UTRAN;
in step 210, the MSC asks for authentication parameters from the HLR;
in step 211, the HLR returns the authentication parameters to the MSC, and the MSC receives the authentication parameters and notifies the UE to perform the authentication process;
in step 212, the authentication is successful;
in step 213, the MSC initiates a location update request to the HLR;
in step 214, the HLR notifies the source MSC to delete the user data;
in step 215, the source MSC notifies the MME to delete the association relationship;
in step 216, the MME returns deletion success message to the source MSC after deleting the user data successfully;
in step 217, the source MSC returns the deletion success message to the HLR;
in step 218, the HLR transmits a request for inserting user data to the MSC;
in step 219, the MSC returns the insertion success message to the HLR after inserting the user data successfully;
in step 220, the HLR returns the location update response message to the MSC;
in step 221, the MSC notifies the UTRAN to perform the encryption process;
in step 222, the MSC returns the location update response message to the UE;
The above steps 208-222 need to be performed only if the MSC to which the UE falls back and the MSC with which the UE jointly registers are not the same.
in step 223, the UE initiates a call establishment request to the MSC;
in step 224, the MSC notifies the UTRAN to establish ground circuits and radio resources;
in step 225, the MSC establishes interoffice bearers to the PSTN.
As described in step 203, the E-UTRAN decides the target cell to which the UE falls back according to the radio signal quality of the measurement report of the UE, which can not ensure the MSC which is served by the target cell to which the UE falls back and the MSC with which the UE jointly registers when accessing the EPS network are the same MSC. When the MSC to which the UE falls back and the MSC with which the UE jointly registers are not the same, the UE initiating a call needs to additionally add steps 208-222, which largely extends the access time of the service.